Method for welding contact plates and contact elements obtained with the method

ABSTRACT

A method for mutually welding a plate that has at least one layer based on Ag alloys and a copper body, comprising the use of laser means for performing the welding, its particularity consisting of the fact that it comprises the steps that consist in:—superimposing and coupling one face of the plate on a surface of the copper body;—starting the welding process by focusing the laser means on a point located proximate to the joint between the face of the plate and the surface of the copper body;—maintaining an angle of incidence of the laser means at values other than 0° with respect to the perpendicular to the surface to be welded;—moving the laser means with respect to the joint, so that the weldpool is pushed along the joint.

The present invention relates to a method for welding contact plates,particularly contact plates having at least one layer based on a silveralloy (such as Ag/Mo, Ag/WC or the like) on a copper or copper-basedelement, particularly on a moving or fixed contact of low-voltagecircuit breakers and contactors. The method according to the presentinvention is based on laser technologies for performing said welding.

Contact plates of circuit breakers and electric contactors, particularlyof low-voltage circuit breakers and contactors, are generally fixed tothe corresponding contact by using braze welding processes with orwithout the addition of metal; another fixing process that can be usedis direct induction or resistance welding.

Although these methods produce satisfactory results, they havedrawbacks, among which mention must certainly be made of the annealingprocess to which the copper is subjected due to the application of heatcaused by the braze welding process. This heating, which can reachtemperatures on the order of 700° C. for a few tens of seconds, causes areduction and degradation of the mechanical characteristics of thecopper.

In order to obviate these drawbacks, attempts have been made to find aprocess that does not produce applications of heat that are harmful forthe mechanical characteristics of the parts to be joined. In particular,patent application EP288585 describes a process for welding contactplates based on laser technologies.

According to what is described in said patent application, the materialthat constitutes the contact plate, which is an alloy of Ag and of ametal oxide, is briefly subjected to fusion at its surface byirradiation with a laser in a reducing environment, preferably hydrogenor a mixture of nitrogen/hydrogen.

In order to reduce reflection phenomena, the surface of the plate isroughened beforehand by brushing or sanding, or is darkened. The laseris then moved so as to cover the entire surface of the plate or so as totrace a grid-like path on it. The contact plates (arranged beforehand onthe component onto which they will be welded) are aligned on a conveyorbelt that conveys them into a chamber that contains a reducingenvironment, under a window that is transparent to laser radiation.

This method, which is based on the conduction of heat on the undersideof the plate by exposing to laser radiation the entire upper surface,despite being possible in theory, is not applied in practice owing toits complexity.

It is evident, from what has been described above, that in the currentart there is the need to have an efficient method for welding contactplates, particularly for welding Ag alloy plates on copper orcopper-based surfaces.

The aim of the present invention is to provide a method for weldingcontact plates that does not require applications of heat that areharmful to the mechanical properties of the components being welded.

Within the scope of this aim, an object of the present invention is toprovide a method for welding contact plates that is based on lasertechnologies and does not require complicated preparation treatments.

Another object of the present invention is to provide a method forwelding contact plates, based on laser technologies, that does notentail the use of reducing environments.

Another object of the present invention is to provide a method forwelding contact plates, based on laser technologies, that providesstable joints between the components subjected to the welding process.

Another object of the present invention is to provide a method forwelding contact plates, based on laser technologies, that can be used inan automated production cycle.

Another object of the present invention is to provide a method forwelding contact plates, based on laser technologies, that ensures massrepeatability.

Another object of the present invention is to provide a method forwelding contact plates, based on laser technologies, that can beindustrialized easily, has modest costs and is economically competitive.

This aim, these objects and others that will become better apparenthereinafter are achieved by means of a method for mutually welding aplate that has at least one layer based on Ag alloys and a copper orcopper-based body, wherein said method comprises the use of laser meansfor performing said welding, characterized in that it comprises thesteps that consist in:

-   -   superimposing and coupling one face of said plate on a surface        of said copper or copper-based body;    -   starting a welding process by focusing the laser means on a        point located proximate to the joint between said face of said        plate and said surface of said copper or copper-based body;    -   maintaining an angle of incidence of said laser means at values        other than 0° with respect to the perpendicular to the surface        to be welded;    -   moving the laser means with respect to the joint, so that the        weldpool is pushed along the joint.

It has been found in fact that by using a welding that entails the useof laser means and by acting according to the method of the presentinvention, a weld is provided between the plate and the copper orcopper-based body which does not have the drawbacks of the braze weldingmethods of the known art and does not require the complicatedrefinements used in laser welding methods of the known art.

Further characteristics and advantages of the present invention willbecome better apparent from the following description of a preferred butnot exclusive embodiment, illustrated only by way of non-limitativeexample in the accompanying drawing, wherein the only FIGURE is aschematic view of a system for mutually welding a plate and a copper orcopper-based body provided according to the invention.

In the accompanying FIG. 1, the plate having at least one layer based onAg alloys is designated by the reference numeral 1, while the copperbody is designated by the reference numeral 2. As mentioned, the methodaccording to the present invention entails, to perform welding, the useof laser means, shown schematically in FIG. 1 and designated by thereference numeral 3.

In order to provide the weld (designated by the reference numeral 5 inthe FIGURE), one face of the plate is superimposed and coupled on asurface of the copper or copper-based body. The welding process isstarted by focusing the laser means on a point located proximate to thejoint between the face of the plate and the surface of the copper body.

In order to avoid or at least minimize reflection phenomena, it isimportant to keep the angle of incidence of the laser means, during thestarting step and during the actual welding process, at values otherthan 0° with respect to the perpendicular to the surface to be welded.In practice, this means that the laser beam must not be perpendicular tothe surface to be welded.

The actual welding process is then performed by moving the laser meanswith respect to the joint so that the weldpool is pushed along thejoint. The “pushed” weldpool condition occurs when a component of theangle of incidence is orientated in the same direction as the relativemotion between the laser beam and the welded joint. It has been foundthat this condition is required in order to allow the weldpool toself-sustain. If one works in “drawn” weldpool conditions (which occurwhen a component of the angle of incidence is oppositely orientated withrespect to the relative motion between the laser beam and the weldedjoint), it has been found that reflections of the laser beam induced bythe plasma increase, generating uncontrolled cycles of quenching andrestarting of the molten weldpool, cratering or dangerous returns of thelaser beam into the generation system.

It is evident to the person skilled in the art that welding is performedby virtue of a relative movement of the laser means with respect to thecomponents to be welded during the welding operation. This relativemovement in practice can be provided by keeping motionless thecomponents to be welded and by moving the laser means, by keeping thelaser means motionless and moving the components to be welded, or bymoving both.

The advancement speed, the angle of incidence and all the other physicalparameters of the laser beam described in greater detail hereinafter canbe chosen and modulated according to the characteristics of the elementsto be welded, such as for example their chemical nature or theirthickness, but can also be controlled and varied appropriately duringthe welding operations in order to compensate for the heating of theregions and in general to optimize the results.

Preferably, the welding process is started by focusing the laser meanson a point of the copper body proximate to the joint between the surfaceof said copper or copper-based body and the face of said plate.

Differently from what one might expect, it has in fact been found thatbetter results are obtained when starting and welding occur at thecopper side of the joint. In particular, it has been found thatparticularly good results are achieved when at least 70% of the moltenmaterial lies, with respect to the joint, on the side that belongs tothe copper body. This condition facilitates the manufacturing processand allows to obtain better mechanical strength characteristics, such astensile strength, than those achieved when working in other manners.

As mentioned, the laser beam must not be perpendicular to the surface tobe welded, and the angle of incidence of the laser means and thereforeof the corresponding laser beam with respect to the perpendicular to thesurface to be welded is between 5 and 20°.

It has been found in practice that it is highly preferable to work withlaser means based on solid-state technologies, for example a Nd crystallaser. In this case also, the characteristics of use of the laser, suchas for example the frequency, power and angle of incidence, may bechosen and modulated even during welding as a function of thecharacteristics of the elements to be welded and of the results to beobtained.

In order to obtain better results, it is preferable for the plate tocomprise, in addition to a layer based on Ag alloys, also a layer ofcopper. Said plates can be obtained for example by means of acoextrusion process and constitute a further aspect of the presentinvention.

By using these plates, the welding process is performed by superimposingand coupling the copper layer of said plate onto the surface of thecopper body.

It has been found in practice that the method according to the inventionsolves the problems of the known art and has many advantages over it,since braze welding techniques and the problems associated therewith areavoided. The combination of characteristics of the method according tothe invention furthermore allows to avoid the phenomena of reflectionand degradation of the weldpool that are typical of processes based onlaser technologies according to the known art.

With the method according to the invention, therefore, it is possible toobtain contact elements that have improved characteristics with respectto the known art. In particular, it is possible to obtain contactelements in which the copper body is constituted by the moving contactor by the fixed contact of a contactor or of a low-voltage circuitbreaker.

The contact elements, as well as the contactors or circuit breakersobtained from them and comprising them, constitute another aspect of thepresent invention.

In practice it has been found that the method according to the inventionand the contact elements obtained thereby fully achieve the intended aimand objects. The invention thus conceived is susceptible of numerousmodifications and variations, all of which are within the scope of theinventive concept. All the details may furthermore be replaced withother technically equivalent elements.

1. A method for mutually welding a plate comprising at least one layerbased on Ag alloys and a copper layer, and a copper body, comprising theuse of a laser means for performing said welding, said methodcomprising: superimposing and coupling the copper layer of said plate ona surface of said copper body; starting welding by focusing said lasermeans on a point located proximate to a joint between one face of saidplate and said surface of said copper body; maintaining an angle ofincidence of said laser means at values that are within a range from 5°to 20° with respect to the perpendicular to said surface of said copperbody; moving the laser means with respect to the joint so as to form anelongated weld, a component of the angle of incidence of said lasermeans being kept oriented along the same direction as the relativemotion between said laser and said welded joint, said laser means beingpointed towards a direction that is opposite with respect to the portionof said joint that has already been welded, thereby forming an elongatedweld having a weld pool that is pushed along said joint to be welded;and maintaining a quantity of molten material of said weld, which lieson a side that belongs to said copper body with respect to said joint,within a range that is equal to or greater than 70%.
 2. The methodaccording to claim 1, wherein starting welding occurs by focusing thelaser means on a point of the copper body proximate to the joint betweenthe surface of said copper body and the face of said plate.
 3. Themethod according to claim 1, wherein said laser means comprise asolid-state laser.
 4. The method according to claim 2, wherein at least70% of molten material formed by welding lies on a side of the jointthat belongs to the copper body.
 5. The method according to claim 2,wherein said laser means comprise a solid-state laser.
 6. The methodaccording to claim 2, wherein said plate comprises at least one copperlayer.
 7. A contact element produced by a method for mutually welding aplate comprising at least one layer based on Ag alloys and a copperlayer, and a copper body, comprising the use of a laser means forperforming said welding, said method comprising: superimposing andcoupling the copper layer of said plate on a surface of said copperbody; starting welding by focusing said laser means on a point locatedproximate to a joint between one face of said plate and said surface ofsaid copper body; maintaining an angle of incidence of said laser meansat values that are within a range from 5° to 20° with respect to theperpendicular to the surface to be welded; and moving the laser meanswith respect to the joint so as to form an elongated weld, a componentof the angle of incidence of said laser means being kept oriented alongthe same direction as the relative motion between said laser means andsaid welded joint, said laser means being pointed towards a directionthat is opposite with respect to the portion of said joint that hasalready been welded, thereby forming an elongated weld having a weldpool that is pushed along said joint to be welded; and maintaining aquantity of molten material of said weld, which lies on a side thatbelongs to said copper body with respect to said joint, within a rangethat is equal to or greater than 70%.
 8. The contact element accordingto claim 7, wherein said copper body is the moving contact of alow-voltage contactor or circuit breaker.
 9. The contact elementaccording to claim 7, wherein said copper body is the fixed contact of alow-voltage contactor or circuit breaker.
 10. A low-voltage circuitbreaker comprising one or more contact elements according to claim 8.11. A low-voltage contactor comprising one or more contact elementsaccording to claim
 8. 12. The method according to claim 7, wherein atleast 70% of molten material formed by welding lies on a side of thejoint that belongs to the copper body.
 13. The method according to claim7, wherein said laser means comprise a solid-state laser.
 14. The methodaccording to claim 7, wherein said plate comprises at least one copperlayer.